Asynchronous communication between various electronic devices, including computers and various peripheral devices, is well known. Such communication is used in a wide variety of technical fields, including specifically relative to this application, communication between two protective relays on a power transmission or distribution line.
In asynchronous communication, which typically uses the EIA-232 (previously RS-232) electrical standard involving the ASCII communication format, the transmitter produces a ground-referenced signal and the receiver measures the signal against ground. The internal clocks of the transmitter and receiver are not synchronized. Metallic cable connects the transmitter and receiver (typically this distance will be less than 50 feet), including at least one wire to transmit the data, one wire to receive data and one ground wire. The transmitted data characters are typically encoded in seven or eight bits, using the ASCII standard. The sequence of data transmission typically includes a start pulse, seven or eight data pulses and a stop pulse.
In certain circumstances, including communication of data, such as for the example given above with respect to protective relays for power systems, time code information is also transmitted between the relays so that the time clocks in the relays and peripheral devices can be synchronized. One time code which is well known and widely used is the IRIG-B time code standard. Under the IRIG-B standard, the demodulated time code is a series of pulses which occur at a 100 pulse per second rate, with the pulse widths being two, five or eight milliseconds. The widest pulses are used to indicate the start and end segments of the transmission of the code, while the narrower pulse widths represent particular binary states. The leading edges of the pulses occur in 10 millisecond intervals, so that the time can be synchronized to 10 milliseconds or better.
When both data and time code information are transmitted, a single metallic cable can be used, although two separate connectors must be used for the data and time code information In some cases, two separate cables are used.
The connecting cables are frequently metallic, which presents a hazard, particularly in high voltage situations. Accordingly, fiber-optic type transmission is being increasingly used to provide the desired electrical isolation. However, in those situations where both time code and data are to be transmitted, those two information streams must be combined in some way or separate fiber-optic cables must be used. Relative to the possibility of combining the information, it is desirable to maintain the existing EIA-232 ASCII and IRIG-B formats, because of their conventionality, and also to use only one channel, if possible.
The two information signals could be combined using conventional multiplexing; however, this would likely be too complex and would require additional hardware, adding to system expense.
The present invention is a multiplexing system sing a single fiber-optic channel, but is relatively simple to implement, without much, if any, additional hardware cost.